Is it possible to use my watch as a compass

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A.� Yes, it is, but it only acts as a rough guide and isn't as accurate as a real compass. Just point where the hour hand is at the sun then look where the 12 o'clock point is, the halfway point between the two indicates where south is. So for example if 2 o'clock is where the sun is then the halfway point, and where south is, is 1 o'clock.

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You can even do this on a cloudy day. Just hold a pencil or stick upright over a piece of paper or light coloured ground and the sun will cast some kind of shadow.

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Q.� How does a magnetic compass work

A.� Imagine a huge bar magnet running from the North Pole to the South Pole within the Earth. Each pole is magnetised. The Earth is surrounded by a huge magnetic field, which causes magnetised needles to swing into a north-south orientation.

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Q.� How accurate is the magnetic compass

A.� That depends on where you are on the Earth. In fact magnetic north and true north do not lie in exactly the same place, but form an angle with each other.

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Q.� Does this variation cause a navigation problem

A.� Not since the introduction of the gyroscopic compass in 1908. This uses a spinning gyroscope that keeps the compass pointing to Earth's true North, rather than magnetic north. Once the gyroscope is set spinning, it remains pointing in the same direction, regardless of the ship's heaving motion.

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Q.� Where was the compass first used

A.� Unfortunately this isn't known although the ancient Greeks certainly knew about magnetic attraction. Very simple magnetic compasses were in use by the 12th century.

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by Lisa Cardy